Digital subscriber line

Digital subscriber line (DSL) (formerly digital subscriber loop) is a family to technologies designed to carrier high-speed transmissions over the telephone company's local loop, specifically the twisted-pair cable infrastructure found in the access portion of the carrier's network. Although it is predominantly associated with Internet services in the mind of the consumer and many in the industry, there are actually many different flavors of DSL that have application in different contexts. What they share in common is the use of a copper-based, twisted-pair cable infrastructure to carry digitized information.

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General Overview

Historically, the telephone companies have used their twisted-pair access infrastructure to deliver plain old telephone service (POTS) to their subscribers. The facility was engineered specifically for the characteristics of human voice. In general, only the voice frequencies (300-3300 Hertz) were used. When engineering was required to enhance the characteristics of these frequencies (e.g., load coils, filters), it was often done at the expense of frequencies other than this narrow set. When subscribers wanted to use this facility for data applications (e.g., dial access to the Internet, Fax), they would place a modem at the end of this channel and use the voice channel for data purposes (i.e., by dialing a connection to another piece of equipment fitted with a modem).

But the twisted-pair cable plant is capable of carrying significantly more (and higher) frequencies. How many frequencies can successfully traverse a given copper loop will depend on the quality of the loop (e.g., number of splices, bridge taps) and its overall length. In general, as the length of the facility increases, the higher frequencies become progressively unusable.

Many of the DSL technologies harvest these higher frequencies by creating additional channels that are approximately 4 kilohertz (KHz) wide. Each channel has a dedicated modem, which automatically adjusts to the best transmission rate it can achieve in that channel (just like the modems we used on the phone lines). These channels are then organized into upstream and downstream groups. Usually, the channels with the highest frequencies flow upstream and the lower frequencies flow downstream. All of the channels that make up the downstream group are then logically bonded together to form a single higher-speed channel.

The twisted-pair cable plant was not just used for POTS services. As the demands of subscribers climbed, and the network became increasingly digital, this same copper was also used to build T-1 systems and (later) to deliver ISDN circuits to customer locations. Variations of DSL have cropped up in both of these contexts, and many others as well.

DSL Variations

There now exist a wide variety of DSL technologies, and the entire group is often referred to as xDSL. Some of the more commonly found DSL types (and each of these has variations within the type) include:

HDSL: A DSL technology most commonly used to deliver DS-1 services over copper.

ADSL: A DSL technology typically used to deliver Internet-access services

SDSL: A DSL technology that provides symmetric speeds, usually offered as a business service